Electrical machines are used throughout a great number of devices today, and typically consist of motors, which convert electrical energy into mechanical energy, and generators, which convert mechanical energy into electrical energy. Generally, electrical machines fall into one of three categories: polyphase synchronous machines, polyphase asynchronous (i.e., induction) machines and direct current (DC) machines. Typical machines consist of two main portions: a stationary, outside portion called a stator, and a rotating, inner portion called a rotor. The rotor of typical machines is mounted on a stiff rod, or shaft, that is supported in bearings so that the rotor is free to turn within the stator to produce mechanical energy.
In one type of synchronous machine, a permanent magnet, brushless direct current (BLDC) machine, the stator is composed of windings that are connected to a controller, and the rotor is composed of two or more permanent magnets of opposed magnetic polarity. The controller includes a driver that generates poly-phase alternating input currents to the stator windings. One conventional driver includes a series of (IGBT's) electrically connected to the phase windings of a BLDC motor. For example, for a three-phase BLDC motor, a conventional driver includes six IGBT's arranged in three half-bridges, where each half-bridge generates a drive for one phase of the motor.
As the rotor rotates within the stator, and the magnets of one polarity approach cores of the stator about which the windings are wound, and that conduct the opposed polarity, sensors signal the angular position of the rotor to the controller which, in turn, controls the alternating currents to switch the polarity of the magnetic field produced by windings of the stator. For example, a three-phase BLDC motor can have two, four or more permanent magnets with alternating magnetic polarities mounted on its rotor. The required rotating magnetic field is produced by current through the stator windings. And the three phases of the current are switched in sequence, which is dictated by the angular position of the rotor.
In many BLDC motor systems, the speed of the BLDC motor is controlled by the driver pulse modulating, such as pulse width modulating, the input voltage generated by the controller. By pulse-width-modulation (PWM) of the input voltage, the driver and, thus, the controller control the average input currents to the windings by using “on” and “off” states. During the time the input currents through the windings are increasing, the voltage supply provides constant voltage to the driver at a level at least as high as the motor voltage required for the desired speed of operation. Once the currents have reached the required levels for the desired speed of the motor, the duty ratio is changed to that required to maintain the currents at or near the required level of current.
Conventional BLDC motor systems that include a driver comprising a series of IGBT's are adequate in controlling the speed of BLDC motors at low frequencies and currents. A standard driver including six IGBT's can drive a three-phase motor (two IGBT's per phase) with a switching frequency up to approximately 20 kHz. In this regard, each IGBT can typically operate with a maximum switching frequency of approximately 20 kHz at a maximum of 50 Amps. Whereas such drivers can control the speed of BLDC motors at low frequencies, such drivers that drive higher power (e.g., greater than one horsepower) and higher voltage (e.g., greater than 200 volts) three-phase motors cannot typically switch at a frequency higher than 20 kHz when the driver comprises IGBT's.
In an attempt to overcome the limit in switching frequency of IGBT's, a driver has been developed that includes high-voltage, high-current metal-oxide semiconductor field-effect transistor (MOSFET's). This driver has a switching rate on the order of 10 ns, and on-resistance (rdson) on the order of 0.004 Ohms. Unfortunately, the MOSFET driver experiences large switching losses. In this regard, MOSFET's used in the driver include inherent diodes (i.e., “body diodes”) that are coupled between the respective MOSFET's source and drain. These body diodes operate with a forward voltage on the order of 70% of the voltage of fast recovery epitaxial diodes (FRED's) used in the driver to absorb the inductive kickback of the BLDC motor phase windings under control. What occurs, then, is a large switching loss due to the slow recovery of the body diodes, resulting in a more than 200% increase in power loss of the driver.
In an effort to reduce the switching loss in the MOSFET driver, one such driver includes MOSFET's arranged in a half-bridge, the half-bridge including FRED's in parallel and blocking diodes in series with the MOSFET's. In operation, the blocking diodes inhibit the body diodes of the MOSFET's from conducting; thereby allowing the FRED's to perform their kickback function with reduced switching loss. But as a tradeoff of realizing reduced switching loss, the MOSFET driver of this configuration experiences increased power loss in the blocking diodes when conducting. This increased power loss can account for a more than 300% increase in the total power lost in an ideal diode-less half-bridge assembly, but less heating of the MOSFET.